AGE & STRUCTURE OF THE CREVICE PLUTON, WEST-CENTRAL IDAHO

The formerly undated Crevice pluton (Blake et al., 2009) outcrops in the lower Salmon River canyon (SRC) ~15 km east of Riggins, Idaho. This leucocratic stock intrudes Precambrian metasedimentary rocks along the ancestral western Laurentian margin, <0.5 km east of the ocean-continent isotopic boundary (e.g., Fleck & Criss, 2004). Together with adjacent biotite-garnet-sillimanite schist, the intrusion is deformed by the mid-Cretaceous western Idaho shear zone (WISZ) and younger superposed structures. Plutonic rocks consist of biotite-granodiorite and granite which locally contain potassium-feldspar megacrysts >15 cm in length. U-Pb zircon ages obtained from the Manning Bridge area using Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) yield 101.8 ± 1.8 Ma silicic magmatism. Latest Early Cretaceous (Albian) crystallization was followed by penetrative deformation, as evidenced by steep gneissic foliation and lineation (WISZ fabric) overprinted by spaced + crenulation cleavage and thin (1-5 m) low-angle ductile shear zones. Mesoscale shears crosscutting the regional gneissosity (WISZ) are variably oriented (N15-70W) but show consistent tops-SW displacement as indicated by imbricated feldspar porphyroclasts, sigma structures, and rare C-S fabrics. Tectonic transport is highly oblique to earlier slip manifested by pervasive down-dip and SE-plunging mineral stretching lineations (both steeply-inclined), and records post-WISZ contraction in the Riggins region. Shallow easterly-dipping brittle faults cut penetrative fabrics in the pluton and elsewhere in SRC (e.g., schistosity at Lake/Rough Creeks 7 km west, where Jura-Cretaceous orogenic belts converge; Hamilton, 1963b, 1969). These late-stage brittle structures are oriented subparallel to mesoscopic shear zones, and locally reactivate (?) the low-angle ductile shears. Differential weathering along NW-trending systematic joints has carved narrow subvertical fins in the stock, recent geomorphic features controlling local drainage patterns and canyon topography. Given its age, composition, and along-strike projection, the Crevice pluton correlates with ca. 105 Ma orthogneiss of the Little Goose Creek complex (Manduca et al., 1993), upon which the WISZ is centered ~25 km south near the village of McCall, Idaho.